FIG. 1 is a block diagram showing a conventional backlight module 10, which includes an AC/DC converter 11, DC-DC voltage converters 121D-12nD, 131D-13nD and 141D-14nD, red light LED (light emitting diode) strings 121-12n, green light LED strings 131-13n, and blue light LED strings 141-14n. The AC/DC converter 11 transforms an AC voltage into a DC voltage. The DC-DC voltage converters 121D-12nD, 131D-13nD, and 141D-14nD, respectively, receive the DC voltage output from the AC/DC converter 11 and transform the DC voltage into operation voltages. The operation voltages from the DC-DC voltage converters are then output to the corresponding LED strings 121-12n, 131-13n, and 141-14n so as to achieve a desired luminance.
In the backlight module 10, each LED string has to be driven by a corresponding DC-DC voltage converter. Generally, the number of LED strings is increased as the panel size is enlarged, and as a result, the number of the DC-DC voltage converters is increased. Consequently, the size of the backlight module 10 is increased, which increases the manufacturing cost of the backlight module.
FIG. 2 is a block diagram showing another example of a conventional backlight module 20. Referring to FIG. 2, the backlight module 20 includes an AC/DC converter 21, a DC-DC voltage converter 22, red light LED strings 231-23n, green light LED strings 241-24n, blue light LED strings 251-25n and constant current controllers 231C-23nC, 241C-24nC and 251C-25nC. The AC/DC converter 21 transforms an AC voltage into a DC voltage, and the DC voltage converter 22 transforms the DC voltage into an operation voltage. During operation, the LED strings 231-23n, 241-24n and 251-25n receive the operation voltage from the DC-DC voltage converter 22, and are respectively coupled to the corresponding constant current controllers 231C-23nC, 241C-24nC and 251C-25nC.
Unlike the backlight module of FIG. 1, the backlight module 20 of FIG. 2 includes just one DC-DC voltage converter 22, which has to drive a relatively large number of the LED strings 231-23n, 241-24n and 251-25n. As a result, the DC-DC voltage converter 22 has to have a relatively large driving capacity, which means that the DC-DC voltage converter 22 is relatively large in size. As a result, the single DC-DC voltage converter 22 consumes a higher amount of power.
Furthermore, in the FIG. 2 implementation, despite the fact that the forward voltages of the red light LED strings 231-23n, the green light LED strings 241-24n and the blue light LED strings 251-25n are different from one another, the same operation voltage is outputted from the single DC-DC voltage converter 22 to drive each of the red light LED strings, the green light LED strings and the blue light LED strings. The forward voltage of an LED is the voltage from the anode to the cathode of the LED at which the LED turns on and starts conducting electricity.
In the arrangement of FIG. 2, when the LED strings turn on, the crossover voltages of the constant current controllers are different from one another. The crossover voltage of a constant current controller is the voltage across the two sides of the constant current controller. The constant current controllers coupled to the LED strings having the lower forward voltages have to withstand a higher crossover voltage, and as a result, the amount of power consumption is higher. In addition, the heat dissipated by the constant current controllers is also increased. Typically, an externally added heat dissipating module has to be provided to lower the temperature, which leads to increased cost.